Myogenesis provides a model system to examine the control of contractile protein biosynthesis. Alpha actin and tropomyosin are found only in muscle, while other polymorphic form of actin and tropomyosin are associated with cell cytoskeleton, movement, division, adhesion, shape and excretion. In the case of muscle alpha actin and tropomyosin, these proteins appear only after myoblast fusion and the controlling factors involved with their genomic regulation is unknown. In order to understand the regulation of contractile protein biosynthesis, I propose to develop 3H cDNA probes to highly purified actin and tropomyosin mRNA isolated from chick breast muscle. These hybridization probes will be utilized to estimate the number of gene copies in DNA and to detect the accumulation of homologous mRNA sequence during muscle development. Because of the similarities in amino acid sequences in the polymorphic forms of actin and/or tropomyosin it will be necessary to demonstrate specificity in hybridization assay. To precisely define the appearance of these RNA species I will manipulate culture conditions by treatment with BudR, EGTA fetal growth medium and conditioned medium. Double stranded cDNA will be used to develop more specific gene probes for alpha actin and tropomyosin mRNA and to determine the location of various base sequences within the structural genes by restriction endonuclease mapping. This is part of a long range goal to eventually isolate the genes which code for actin and tropomyosin and to determine the following: The earliest appearance of these gene sequences in nuclei and cytoplasm, the precursor relationship in heavy nuclear RNA, and the lifetime or stability of these mRNA during myogenesis. The utilization of these probes to actin and tropomyosin may provide the tools to unravel the complex mechanisms involved in gene regulation during the induction of muscle protein synthesis.